In any compartment, there is always the possibility of penetration from a projectile. When this occurs, such as in the case of a self-sealing fuel tank, some amount of fuel is lost through the penetration hole before the sealing material can seal the hole against further leakage. In other situations, the projectile can strike a surface in the penetrated compartment and the friction or flash of the strike can start a fire in the compartment. Passage of the projectile into the compartment therefore poses a significant fire risk.
In many vessels, such as in aircraft, compartments adjacent to fuel tanks may carry important steering, navigational, and control components that could be greatly damaged by a fire started when a bullet crashes through the walls of the fuel tank and into the adjacent compartment. In other situations, the mere entry of the projectile and striking of one of the components can cause a fire that can possibly do more damage than the initial penetration and collision with the equipment. The projectile, such as a bullet, is usually hot from friction generated in the penetration portion of the collision. This is even further of importance where the projectile contains an incendiary compound whose sole purpose is to start a fire inside the compartment that it has penetrated.
In aircraft, it has been the practice to cover the inside surface of compartments with panels containing a frangible honeycomb structure filled with a fire suppressant substance such as aluminum oxide. The operation of this fire mitigation technique begins when the projectile enters the compartment. Passage of the projectile through the wall causes collapse of the honeycombs (or cells) in the area of penetration and the honeycombs release their aluminum oxide powder that is carried by the turbulence produced by the projectile along its path of entry. The powder interferes with air reaching burnable substances (fuel) and acts to prevent a fire from starting as well as extinguishing any existing fire.
Often the size of the hole caused by the projectile is small, thereby breaking open only a small number of cells and releasing only a small amount of fire suppressant substance. This allows very little fire suppressant to be available to follow the leaked fuel or reach the flammable substances in the compartment. The slimmer the projectile and the higher its velocity, the less energy is lost in its penetration of the compartment and the smaller the hole in the panel.
It was thought that a layer of a small amount of the pyrotechnic compound BTATz (3,6-BIS(1H-1,2,3,4-TETRAZOL-5-YLAMINO)-1,2,4,5-TETRAZINE) could be placed on a supporting backing panel on which the powder panel was affixed, thereby exploding and dispersing the fire suppressant powder in case of projectile impact. (See Bennett, Aircraft Survivability, Fall 2002, p. 14-15). Subsequently it has been demonstrated that BTATz was unsuitable for exploding and dispersing the fire suppressant powder.
U.S. Pat. No. 6,657,059 for 3,6-BIS(1H-1,2,3,4-TETRAZOL-5-YLAMINO)-1,2,4,5-TETRAZINE (BTATz) or Salts thereof, which issued on Dec. 02, 2003 to Michael Hiskey et al. describes uses for BATz and its salts. Specifically, U.S. Pat. No. 6,657,059 describes BTATz as being a very energetic fuel containing no oxygen in its structure. This Patent further states that it has been found that a pressed pellet of 0.5 inch diameter BTATz does not detonate, and that BTATz has desirable properties for a propellent material making BTATz very useful as a solid rocket motor propellent allowing a rocket motor designer leeway. U.S. Pat. No. 6,657,059 includes seven examples describing BTATz compounds and properties. Example seven in U.S. Pat. No. 6,657,059 clearly states that BTATz has both a high burn rat and a low pressure component making a suitable candidate as a high performance propellent fuel.
An article entitiled “Tetrazine Explosives” appearing in the Journal of Propellents, Explosives, Pyrotechnics 29 (2004), No. 4, pages 209-215 describes various properties of BTATz. In this article BTATz is described as being non-detonable even when boosted by another explosive (PBX 9501) as well as having a poorly defined sensitivity to impact.
BTATz burns rapidly releasing nitrogen gas. However, the reaction rate of BTATz is not fast enough to pressurize a panel and shatter a large area of the panel. The decomposition of BTATz emanates linearly from an impact point which assumes that the impact would set it off which is doubtful. Potentially all of the gas generated could escape through the impact point which would result in no further damage to the panel or only lead to limited tearing about the impact point.
When gases are not generated quickly enough they dissipate along the same non-obstructed path resulting in a less effective disruption of the panel. Therefore the process discussed by Bennett using BTATz would not result in more effective fire suppression.
Accordingly, there is a need for an efficient and effective means for increasing the amount of fire suppressant substance dispersed in a compartment upon the penetration of a projectile.